The present invention relates to a movable blade of a turbine and in particular, to a turbine movable blade having a leading edge of an improved structure.
Normally, a steam turbine comprises a plurality of stages each provided by combining a turbine stationary blade, which may be called turbine nozzle, and a turbine movable blade, which may be called turbine bucket. A plurality of stages are arranged on a turbine shaft (rotor) along the stream of a turbine driving steam (main stream) and the turbine stationary and movable blades are arranged as an annual sequence along the circumferential direction of the turbine shaft.
Among the plural stages arranged on the turbine shaft, the final stage is driven by wet steam which has lost thermal energy and contains large quantity of droplets, and there may cause a problem of an erosion. The movable blade, in particular, directly receives droplet containing wet steam splashed from the stationary blade arranged upstream thereof. The wet steam may cause erosion to the leading edge thereof, i.e. front side edge with respect to a turbine steam flow. Since the movable blade in the final stage is as long as about 1 m compared with those in the remaining stages and has a greater centrifugal force and vibration stress, if an erosion occurs, there is a fear that a stable operation may be badly affected by the synergistic effect thereof.
FIG. 7 shows a perspective view of a conventional steam turbine, partially cut away, having a plurality of pressure stages to which the present invention is applicable. The shown steam turbine includes a turbine rotor as a rotational shaft 100 around which a plurality of pressure stages such as high pressure stage 101 and low pressure stage 102 (103) are formed. As is generally known in this art field, each pressure stage includes stationary and movable blade unit which are mounted circumstantially to the turbine rotor. The details of the arrangement of these members are well known in the art and detail description thereof is omitted herein.
FIG. 5 shows a conventional movable blade of a turbine, such as shown in FIG. 7, under the strict environment as stated above has been intended to prevent erosion caused by the wet steam, by attaching, for example, welding an erosion shield plate 3 made of high erosion protective material such as, for example, cobalt group alloy steel onto a leading edge 2 of the main body of the movable blade 1 (called hereinlater movable blade body 1 or merely blade body 1) with silver brazing or welding. That is, a portion of the leading edge of the movable blade body is formed to have a recessed portion into which the erosion shield piece is welded as a separate member.
However, when such silver brazing or welding is applied, a connection defect occurs to or a residual stress remain in the base material of the blade body 1. After the movable blade has been used for a long period of time, the erosion shield plate 3 may be peeled off. Besides, if the erosion shield plate is replaced repeatedly, various problems including occurrence of fatigue to the base material of the blade body I are caused due to a thermal effect. Considering this, the following measures have been taken for the recent movable blade as shown in FIG. 6, which is also applicable to a steam turbine of FIG. 7. When the erosion shield piece 3a is attached to the leading edge 2 of the blade body 1, surface flame hardening is locally conducted to the blade body 1 and the erosion shield piece 3a to maintain high strength, thereby preventing erosion by the wet steam.
As described above, in the recent turbine movable blade, the surface flame hardening is conducted instead of junction means such as welding when the erosion shield piece 3a is attached to the leading edge 2 of the blade body 1. Accordingly, welding defects, liftoff and the like can be suppressed and the movable blade can be thus operated while maintaining the quality thereof in a stable condition even under the severe circumstances.
Normally, 12-chromium alloy steel is often used for the base material of the blade body 1 in the movable blade. In that case, since there are portions subjected to flame hardening and portions not subjected to the flame hardening between the erosion shield piece 3a and the 12-chromium alloy steel, a residual tensile force might sometimes occur. For this reason, the portions of the movable blade body subjected to the flame hardening are often cracked by stress corrosion crack or fatigue, which has made it difficult to maintain the quality of the movable blade in a stable condition.
To suppress the residual tensile stress generated at the 12-chromium alloy steel, there is proposed, for example, conducting of a shot peening to portions subjected to the flame hardening. This, however, is not a durable measure to maintain stable quality of the movable blade body.